Focal point adjusting method, and optical pickup device

ABSTRACT

A focus control process (S 2 ), a spherical aberration correcting process (S 4 ), and a focus offset adjusting process (S 7  to S 9 ) are performed in this order. In the focus control process, an output of a focus error signal obtained by detecting focal point displacement that occurs in a direction of an optical axis of a light beam focused through a two-element object lens is controlled so that the output becomes close to zero. In the spherical aberration correcting process, spherical aberration that occurs with respect to the light beam is corrected. In the focus offset adjusting process, offset of the focus error signal is adjusted. In this way, it is possible to provide a focal point adjusting method and an optical pickup device in which focus control can be performed stably by eliminating the offset.

TECHNICAL FIELD

The present invention relates to a focal point adjusting method fordetecting focal point displacement caused by a focusing optical systemand adjusting a focal point, and relates to an optical pickup deviceemploying the focal point adjusting method.

BACKGROUND ART

It is a recent demand to increase storage density of an optical disk, soas to respond to an increasing amount of information. Under thiscircumstance, linear storage density of an information storing layer inan optical disk has been increased, and a pitch of tracks has beennarrowed, so as to increase the storage density of an optical disk. Inorder to respond to the increased storage density of an optical disk, itis necessary to reduce a beam diameter of a light beam focused on aninformation storing layer of the optical disk.

Possible methods for reducing the beam diameter of the light beam are asfollows: (1) increasing a numerical aperture (NA) of the light beamradiated from an object lens, which is a focusing optical system of anoptical pickup device that performs recording and reproduction by usingthe optical disk, and/or (2) shortening a wavelength of the light beam.

It is conceivable that the wavelength of the light beam can be decreasedby changing a light source, that is, by replacing a red semiconductorlaser with a bluish-purple semiconductor laser, which is beginning to beproduced commercially on a large scale.

On the other hand, as a method for realizing an object lens having ahigh numerical aperture, proposed is a method in which the object lensis combined with a hemispherical lens. In this method, an object lensincludes the two lenses (two lenses in group).

Generally, an optical disk is arranged so that an information storinglayer thereof is covered with a cover glass. This is to protect theinformation storing layer from dust and scratch. Therefore, a light beamtransmitted through an object lens of an optical pickup device istransmitted through the cover glass, and focused on the informationstoring layer under the cover glass. In this way, a focal point is made.

When the light beam is thus transmitted through the cover glass,spherical aberration (SA) is caused. The spherical aberration SA isgiven bySA∝d·NA⁴  (1).The spherical aberration SA is proportional to a thickness d of thecover glass and to a fourth power of the numerical aperture NA of theobject lens. Usually, the spherical aberration of the light beamtransmitted through the object lens and the cover glass is sufficientlysmall. This is because the object lens is usually designed so that thespherical aberration is canceled out.

However, if the thickness of the cover glass is scooched from apredetermined value, spherical aberration is caused with respect to thelight beam focused on the information storing layer. This increases thebeam diameter. As a result, there is a problem that information cannotbe read and written correctly.

According to Formula (1), an error ΔSA of the spherical aberrationcaused by an error Δd of the thickness of the cover glass isproportional to the error Δd of the thickness of the cover glass. Thatis, the greater the Δd of the thickness of the cover glass is, thegreater is the error ΔSA of the spherical aberration. Because of this,information cannot be read and written correctly.

In case of a conventional optical disk such as a DVD (Digital VersatileDisc), a numerical aperture NA of an object lens of an optical pickupdevice used is small: about 0.6. Therefore, the error ΔSA of thespherical aberration caused by the error Δd of the thickness of thecover glass is small. As a result, it is possible to focus the lightbeam as a sufficiently small spot on each information storing layer.

Meanwhile, a DVD including two information storing layers is alreadyproduced commercially. Such a DVD is an example of a multi-layeredoptical disk in which information storing layers are laminated so as toincrease information storage density in a thickness direction of theoptical disk. In an optical pickup device that performs recording andreproduction by using such a multi-layered optical disk, it is necessaryto focus the light beam as a sufficiently small spot on each informationstoring surface of the optical disk.

However, in such a multi-layered optical disk, a thickness from asurface of the optical disk (surface of the cover glass) to theinformation storing layer is different from layer to layer. Therefore,spherical aberration caused when the light beam is transmitted throughthe cover glass of the optical disk is different from layer to layer.Here, for example, a difference (error ΔSA) between spherical aberrationoccurring in adjacent information storing layers is proportional to aninterlayer distance t (equivalent to the thickness d) between theadjacent information storing layers, according to Formula (1).

Even if the error Δd of the thickness of the cover glass is the same, ahigher numerical aperture NA results in greater spherical aberration SA.For example, if NA=0.85, the spherical aberration SA is approximatelyfour times greater than that of a case in which NA=0.6. Therefore, fromFormula (1), it is found that the higher the numerical aperture is, suchas when NA=0.85, the greater is the spherical aberration caused by theerror Ad of the thickness of the cover glass.

Likewise, in case of the multi-layered optical disk, even if theinterlayer distance between adjacent information storing layers is thesame, a higher NA of the object lens of the optical pickup deviceresults in a greater difference (error ΔSA) in spherical aberration. Forexample, if NA=0.85, an error in the spherical aberration SA isapproximately four times greater than that of a case in which NA=0.6.Therefore, from Formula (1), it is found that the higher the numericalaperture is, such as when NA=0.85, the greater is the difference inspherical aberration between information storing layers.

Therefore, an object lens having a high numerical aperture isproblematic in that (1) an error in spherical aberration has anunignorable influence, and (2) information is read with lower accuracy.In view of the circumstance, it is necessary to correct the sphericalaberration in order to attain high storage density by using an objectlens having a high numerical aperture.

As a means of correcting the spherical aberration, an optical pickupdevice that detects and corrects the spherical aberration is disclosedin Japanese Publication For Unexamined Patent Application, Tokukai2000-171346 (publication date: Jun. 23, 2000), for example. The opticalpickup device makes use of a phenomenon that, when light is focused onan information storing layer of an optical disk, a light beam in avicinity of an optical axis of the light and a light beam exterior tothe vicinity of the optical axis are focused on different focuspositions, due to the spherical aberration.

According to the optical pickup device disclosed in the publication, thelight beam to be detected is split, into a light beam in the vicinity ofthe optical axis and a light beam exterior to the vicinity of theoptical axis, by an optical element such as a hologram. Then, whenspherical aberration is caused, with respect to one of the light beams,a gap between the focus position and the information storing layer isdetected. Based on a result of the detection, the spherical aberrationis corrected. In this way, it is possible to sufficiently reducediameters of the light beams respectively focused on the informationstoring layers of the optical disk.

In accordance with an amount of the spherical aberration that is thusdetected, it is possible to correct the spherical aberration of afocusing optical system of the optical pickup device by using aspherical aberration correcting mechanism. In this way, it is possibleto keep the spherical aberration always small. Moreover, by performing,while optical information is recorded or reproduced, a sphericalaberration correcting servo in which spherical aberration is detectedand corrected so that an amount of the spherical aberration is alwayskept small, it is possible to keep the beams always in a best conditionwhile information is recorded in or reproduced from a magnetoopticalstoring medium.

However, the optical pickup device disclosed in the publication includesnot only (1) a focusing servo, which ensures that a focus is always onthe information storing layer, and (2) a tracking servo, which focuses alight beam on a central position of a track of an optical informationstoring medium, but also (3) a spherical aberration correcting servo.

Therefore, if a servo introduction order and offset adjustment for aservo signal are inadequate, offset is left in the servo signal.

DISCLOSURE OF INVENTION

The present invention was made in light of the foregoing problems. Anobject of the present invention is to provide a focal point adjustingmethod and an optical pickup device that perform stable focus control byeliminating offset.

To solve the foregoing problems, a focal point adjusting method of thepresent invention for adjusting a focal point of a focused light beamincludes:

a focus controlling process in which an output of a focus error signalis controlled so that the output becomes close to zero, the focus errorsignal being obtained by detecting focal point displacement that occursin a direction of an optical axis of the light beam focused by passingthrough a focusing optical system;

a spherical aberration correcting process in which spherical aberrationthat occurs with respect to the light beam is corrected; and

a focus offset adjusting process in which offset of the focus errorsignal is adjusted.

In this method, the offset of the focusing error signal is adjustedafter (i) the output of the focus error signal is reduced to zero by thefocus controlling process, and (ii) a slope of a linear portion of thefocus error signal is steepened so that the spherical aberration thatoccurs in the focusing optical system becomes so small as to beignorable.

Therefore, if the offset of the focus error signal is adjusted after thespherical aberration is corrected with respect to the focus error signalwhose output is zero, the offset is adjusted to be zero after the slopeof the linear portion of the focus error signal is steepened.

Thus, offset can be eliminated from the focus error signal by adjustingthe offset of the focus error signal after correcting the sphericalaberration and, for example, turning ON a loop of a spherical aberrationcorrecting servo, so that the spherical aberration, which is generatedin the focusing optical system, is reduced to a smallest possibleamount.

As a result, it is possible to provide a focal point adjustment methodwith which the spherical aberration is corrected stably, and the focalpoint displacement that occurs in the direction of the optical axis iscontrolled stably, so that the focal point of the light beam radiatedwill not be displaced, for example.

Moreover, it is preferable that, in the focal point adjusting method,the spherical aberration is detected from a focus error signal obtainedfrom at least one of (a) an inner circumferential region of the lightbeam and (b) an outer circumferential region of the light beam, whichare split from each other by light beam splitting means.

With this arrangement, it is possible to detect the spherical aberrationby detecting a difference of (1) a focal point of the innercircumferential region of the light beam or (2) a focal point of theouter circumferential region of the light beam. Therefore, it ispossible to detect the spherical aberration sensitively.

Alternatively, it is preferable that, in the focal point adjustingmethod of the present invention, a spherical aberration error signalSAES, which is indicative of the spherical aberration, satisfies any oneof the following:SAES=F1−(F1+F2)×K1;SAES=F2−(F1+F2)×K2; andSAES=F1−F2×K3,where F1 is a first focus error signal obtained by detecting the focalpoint displacement, which occurs in the direction of the optical axis,of the outer circumferential region of the light beam, F2 is a secondfocus error signal obtained by detecting the focal point displacement,which occurs in the direction of the optical axis, of the innercircumferential region of the light beam, and K1, K2, and K3 arecoefficients.

With method, it is possible to remove, from the spherical aberrationerror signal SAES, crosstalk from the focus error signal. Therefore, itis possible to detect the spherical aberration accurately from thespherical aberration error signal SAES.

It is preferable that, in the focal error adjusting method of thepresent invention, the spherical aberration of the focusing opticalsystem is corrected by moving at least one lens of a lens groupincluding one or more lens of the focusing optical system.

With this method, it is possible to correct the spherical aberrationaccurately with a simple arrangement.

It is preferable that, in the focal point adjusting method of thepresent invention, there is a repetition of the spherical aberrationcorrecting process and the focus offset adjusting process; and a focalpoint adjustment for the light beam is terminated after the focus offsetadjustment process is performed at an end of the repetition.

With this method, by repeating the spherical aberration correctingprocess and the focus offset adjusting process, it is possible tocorrect the spherical aberration and to adjust the offset of the focuserror signal even if offset amount of the focus error signal and aremaining amount of the spherical aberration are initially (i.e. beforethe focal point adjustment is performed) large.

When the spherical aberration is corrected, a sensitivity of the focuserror signal is changed. This change causes offset. However, accordingto the arrangement above, the focal point adjustment is not terminatedin the presence of such offset, because the focus offset adjustmentprocess is performed at the end of the repetition.

Therefore, it is possible to terminate the focal point adjustment afteroffset has been eliminated. As a result, it is possible, for example, toreproduce information from the optical storing medium without offset.

Moreover, an optical pickup device of the present invention includes:

a light source;

a focusing optical system that focuses a light beam radiated from thelight source and reflected on a storing medium;

focusing error detecting means for detecting a focus error signalindicative of focal point displacement that occurs in a direction of anoptical axis of the light beam;

focus control means for controlling an output of the focus error signalso that the output becomes close to zero;

focus offset adjusting means for adjusting offset of the focus errorsignal;

spherical aberration detecting means for detecting spherical aberrationof the focusing optical system; and

spherical aberration correcting means for correcting the sphericalaberration,

the focusing offset adjusting means adjusting the offset of the focuserror signal after (i) the focus control means controls the output ofthe focus error signal, and (ii) the spherical aberration correctingmeans corrects the spherical aberration.

In this arrangement, the offset of the focusing error signal is adjustedafter (i) the output of the focus error signal is reduced to zero by thefocus control means, and (ii) a slope of a linear portion of the focuserror signal is steepened so that the spherical aberration becomes sosmall as to be ignorable.

Thus, offset can be eliminated from the focus error signal by adjustingthe offset of the focus error signal after (a) correcting the sphericalaberration and, for example, (b) turning ON a loop of the sphericalaberration correcting servo, so that the spherical aberration, whichoccurs in the focusing optical system, is reduced to a smallest possibleamount.

As a result, it is possible to provide an optical pickup device withwhich the spherical aberration is corrected stably and the focal pointdisplacement that occurs in the direction of the optical axis iscontrolled stably, so that the focal point of the light beam radiatedwill not be displaced.

Alternatively, it is preferable that the optical pickup device of thepresent invention further includes:

light beam splitting means for splitting the light beam transmittedthrough the focusing optical system, into (a) an inner circumferentialregion and (b) an outer circumferential region,

the spherical aberration detecting means detecting the sphericalaberration from a focus error signal obtained from at least one of (a)the inner circumferential region of the light beam and (b) the outercircumferential region of the light beam.

With this arrangement, it is possible to detect the spherical aberrationby detecting a difference of (1) the focal point of the innercircumferential region of the light beam or (2) the focal point of theouter circumferential region of the light beam. Therefore, it ispossible to detect the spherical aberration sensitively.

In is preferable that, in the optical pickup device of the presentinvention, the spherical aberration detecting means generates aspherical aberration error signal indicative of the spherical aberrationof the focusing optical system; and SAES, which is the sphericalaberration error signal, satisfies any one of the following:SAES=F1−(F1+F2)×K1;SAES=F2−(F1+F2)×K2; andSAES=F1−F2×K3,where F1 is a first focus error signal obtained by detecting the focalpoint displacement, which occurs in the direction of the optical axis,of the outer circumferential region of the light beam, F2 is a secondfocus error signal obtained by detecting the focal point displacement,which occurs in the direction of the optical axis, of the innercircumferential region of the light beam, and K1, K2, and K3 arecoefficients.

With this arrangement, it is possible to remove, from the sphericalaberration error signal SAES, crosstalk from the focus error signal.Therefore, it is possible to detect the spherical aberration accuratelyfrom the spherical aberration error signal SAES.

It is preferable that, in the optical pickup device of the presentinvention, the spherical aberration correcting means corrects thespherical aberration by performing such an adjustment as to maximize anamplitude of a reproduction signal obtained by reading informationstored in the storing medium.

With this arrangement, it is possible to correct the sphericalaberration by monitoring the reproduction signal, and driving thefocusing optical system so that the amplitude of the reproduction signalis maximized. Therefore, the spherical aberration can be correctedaccurately with a simple arrangement.

It is preferable that, in the optical pickup device of the presentinvention, the spherical aberration correcting means corrects thespherical aberration by performing such an adjustment as to maximize anamplitude of a tracking error signal indicative of the focal pointdisplacement that occurs in a radial direction of the storing medium.

With this arrangement, it is possible to correct the sphericalaberration by monitoring the tracking error signal, and driving thefocusing optical system so that the amplitude of the tracking errorsignal is maximized. Therefore, the spherical aberration can becorrected accurately with a simple arrangement.

Moreover, it is preferable that, in the optical pickup device of thepresent invention, the focusing optical system is a lens group includingone or more lens; and the spherical aberration correcting means moves atleast one lens of the lens group.

With this arrangement, it is possible to correct the sphericalaberration accurately with a simple arrangement.

It is preferable that, in the optical pickup device of the presentinvention, the focus offset adjusting means adjusts the offset of thefocus error signal by performing such an adjustment as to maximize anamplitude of a reproduction signal obtained by reading informationstored in the storing medium.

With this arrangement, the offset of the focus error signal can beadjusted, by monitoring the reproduction signal, and driving thefocusing optical system, for example, so that the amplitude of thereproduction signal is maximized. Therefore, the offset can be adjustedwith high accuracy.

It is preferable that the optical pickup device of the present inventionfurther includes:

a tracking control means for (a) detecting a tracking error signalindicative of focal point displacement that occurs in a radial directionof the storing medium, and (b) correcting, in accordance with thetracking error signal, the focal point displacement that occurs in theradial direction of the storing medium,

the focus offset adjusting means adjusting the offset of the focus errorsignal whose focal point displacement that occurs in the radialdirection of the storing medium has been adjusted by the trackingcontrol means.

With this arrangement, it is possible to prevent the amplitude of thereproduction signal from being changed by an influence of the trackingerror signal. Therefore, the offset of the focus error signal can beadjusted with high accuracy.

It is preferable that the optical pickup device of the present inventionfurther includes:

a tracking control means for (a) detecting a tracking error signalindicative of focal point displacement that occurs in a radial directionof the storing medium, and (b) correcting, in accordance with thetracking error signal, the focal point displacement that occurs in theradial direction of the storing medium,

the focus offset adjusting means adjusting the offset of the focus errorsignal by performing such an adjustment as to maximize an amplitude ofthe tracking error signal.

With this arrangement, even if the reproduction signal cannot be usedfor the offset adjustment because no modulated component appears in thereproduction signal of an unused storing medium (e.g. if the storingmedium has a wobble structure, that is, if the storing medium hasundulating track grooves so as to store address information), it ispossible to adjust the offset of the focus error signal.

It is preferable that, in the optical pickup device of the presentinvention, if (a) the storing medium has a plurality of informationstoring layers, and (b) information is recorded in and reproduced fromthe storing medium, the focus offset adjusting means adjusts the offsetwhen the focal point of the light beam jumps from one of the informationstoring layers into another of the information storing layers.

With this arrangement, if the storing medium has a plurality ofinformation storing layers, the offset can be removed from the focuserror signal not only when the storing medium is loaded, but also whenthe focal point of the light beam jumps from one of the informationstoring layers into another of the information storing layers.

It is preferable that, in the optical pickup device of the presentinvention, if there is a repetition of (a) offset adjustment for thefocus error signal by the focus offset adjusting means and (b) acorrection of the spherical aberration, the focus offset adjusting meansperforms, at an end of the repetition, (a) the offset adjustment for thefocus error signal.

With this arrangement, by repeating the offset adjustment and thecorrection of the spherical aberration, it is possible to correct thespherical aberration and to adjust the offset of the focus error signaleven if the offset amount of the focus error signal and the remainingamount of the spherical aberration are initially (i.e. before the focalpoint adjustment is performed) large.

When the spherical aberration is corrected, the sensitivity of the focuserror signal is changed. This change causes offset. However, accordingto the arrangement above, the focal point adjustment will not beterminated in the presence of such offset, because the focus offsetadjustment process is performed at the end of the repetition.

Therefore, it is possible to terminate the focal point adjustment afterthe offset has been eliminated. As a result, it is possible, forexample, to reproduce information from the optical storing mediumwithout offset.

The following description will sufficiently explain other objectives,features, and advantages of the present invention. Benefits of thepresent invention will be clearly explained below, with reference to theattached drawings.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 illustrates one embodiment of the present invention. In FIG. 1, aschematic arrangement of an optical recording and reproducing apparatusis illustrated. Here, the optical recording and reproducing apparatusincludes an optical pickup device that employs a focal point adjustmentmethod.

FIG. 2 illustrates an arrangement of a substantial part of the opticalrecording and reproducing apparatus including the optical pickup deviceof FIG. 1.

FIG. 3 is a detail view illustrating a detecting device of the opticalpickup device of FIG. 1.

FIG. 4 is a flowchart illustrating a procedure of the focal pointadjusting method for a two-element object lens.

FIG. 5 is a flowchart illustrating a procedure of a focal pointadjusting method for a two-element object lens in a comparative example.

FIG. 6( a) is a graph illustrating a relationship between (1) a focuserror signal FES and (2) a defocus amount, before spherical aberrationis corrected. FIG. 6( b) is a graph illustrating a relationship between(1) the focus error signal FES and (2) the defocus amount, after thespherical aberration is corrected.

FIG. 7 is a flowchart illustrating another procedure of a focal pointadjusting method for a two-element object lens.

FIG. 8 is a flowchart illustrating an example of a procedure of thefocal point adjusting method for the two-element object lens. Thisexample is for a case in which an RF signal is used for correcting thespherical aberration.

FIG. 9 is a flowchart illustrating an example of a procedure of thefocal point adjusting method for the two-element object lens. Thisexample is for a case in which a tracking error signal TES is used forcorrecting the spherical aberration.

BEST MODE FOR CARRYING OUT THE INVENTION

The following embodiments and comparative examples more specificallydescribe the present invention. It should be noted that the presentinvention is not limited to the embodiments and comparative examples.

With reference to FIGS. 1 to 9, one embodiment of the present inventionis described below. Note that the present embodiment discusses anexample in which a focal point adjusting method of the present inventionis employed in an optical pickup device provided to an optical recordingand reproducing apparatus that records and reproduces informationoptically by using an optical disk (storing medium), which is an opticalstoring medium.

As shown in FIG. 1, the optical recording and reproducing apparatus ofthe present embodiment includes an optical pickup device 10 and aspindle motor 21. The optical pickup device 10 includes a drive controlsection 30. The optical recording and reproducing apparatus records andreproduces information to and from an optical disk (storing medium) 6.

The spindle motor 21 drives the optical disk 6 so as to rotate theoptical disk 6. Note that the optical disk 6 may be of any kind (e.g. amagnetooptical disk) with no particular limitation, as long as theoptical disk 6 is an optical disk.

The optical pickup device 10 radiates a light beam into the optical disk6, so as to record and reproduce information to and from the opticaldisk 6. The optical pickup device 6 includes a semiconductor laser(light source) 1, a hologram (light beam splitting means) 2, acollimating lens 3, a two-element object lens (focusing optical system)9, a mirror 27, and detecting devices 7 and 8.

The semiconductor laser 1 is a light source that emits a light beam soas to radiate the light beam into the optical disk 6. Note that thelight beam emitted from the semiconductor laser 1 may have anywavelength, with no particular limitation.

The collimating lens 3 converts, into parallel light, the light beamemitted from the semiconductor laser 1 and transmitted through thehologram 2 as a zero-order diffracted light beam.

The two-element object lens 9 includes a first element 4 and a secondelement 5, which are object lenses provided in this order from a sidefrom which the semiconductor laser 1 radiates the light beam. The firstelement 4 is held at peripheral portions thereof by a holder 22.Moreover, at outer circumferential portions of the holder 22, there areprovided a focus actuator (focus control means; focus offset adjustingmeans) 23, and a tracking actuator (tracking control means) 26.

The focus actuator 23 performs focus control by moving the holder 22 ina direction of an optical axis, so as to lead the two-element objectlens 9 into an appropriate position.

Driving of the tracking actuator 26 is controlled so that the holder 22is moved in a radial direction (that is, a direction perpendicular to adirection of tracks formed on the optical disk 6 and to the opticalaxis; i.e. a direction of a radial of the optical disk 6). In this way,it is possible to cause the light beam to trace an information track onthe optical disk 6 accurately.

The second element 5 is held at peripheral portions thereof by a holder24. Moreover, at outer circumferential portions of the holder 24, thereis an actuator (spherical aberration correcting means) 25. Driving ofthe actuator 25 is controlled so that a distance between the firstelement 4 and the second element 5 is adjusted. In this way, it ispossible to correct spherical aberration that occurs in the opticalsystem of the optical pickup device 10.

The mirror 27 is provided between the two-element object lens 9 and thecollimating lens 3. The mirror 27 deflects, by approximately 90°, anoptical path of the light beam from the two-element object lens 9, or anoptical path of a light beam from the collimating lens 3.

Each of the detecting devices 7 and 8 includes a plurality of lightreceiving elements (light receiving sections). In order to outputsignals such as a track error signal, the detecting devices 7 and 8convert light beams respectively received by the receiving elements intoelectrical signals. Details of the optical pickup device 10 aredescribed later.

The drive control section 30 controls driving of the spindle motor 21and the optical pickup device 10. The drive control section 30 includesa spindle motor drive circuit 31, a focus drive circuit (focus controlmeans) 33, a tracking drive circuit (tracking control means) 34, asecond element drive circuit (spherical error correcting means) 32, acontrol signal generating circuit (focus error detecting means;spherical aberration detecting means) 35, and an information reproducingcircuit 36.

The control signal generating circuit 35 is a control signal generatingcircuit that generates, from the signals supplied from the detectingdevices 7 and 8, control signals for the control circuits.

Specifically, in accordance with the signals supplied from the detectingdevices 7 and 8, the control signal generating circuit 35 generates atracking error signal TES, a focus error signal FES, and a sphericalaberration error signal SAES, which are described later. The trackingerror signal TES is supplied to the tracking drive circuit 34. The focuserror signal FES is supplied to the focus drive circuit 33. Thespherical aberration error signal SAES is supplied to the second elementdrive circuit 32. Then, in accordance with the signals supplied, thedrive circuits respectively control driving of members.

The spindle motor drive circuit 31 controls the driving of the spindlemotor 21 in accordance with a signal supplied from the control signalgenerating circuit 35.

The focus drive circuit 33 controls driving of the focus actuator 23 inaccordance with the focus error signal FES generated by the controlsignal generating circuit 35. For example, the driving of the focusactuator 23 is controlled so that, when the focus error signal FES issupplied to the focus drive circuit 33, the two-element object lens ismoved in the direction of the optical axis in accordance with a value ofthe focus error signal FES. In this way, focal point displacement of thetwo-element object lens that occurs in the direction of the optical axisis corrected.

The tracking drive circuit 34 controls driving of the tracking actuator26 in accordance with the tracking error signal TES generated by thecontrol signal generating circuit 35.

The second element drive circuit 32 controls driving of the actuator 25in accordance with the spherical aberration error signal SAES generatedby the control signal generating circuit 35. For example, the driving ofthe actuator 25 is controlled as follows. When the spherical aberrationerror signal SAES is supplied to the second element drive circuit 32,the second element (lens) 5 is moved in the direction of the opticalaxis in accordance with a value of the spherical aberration error signalSAES so that the spherical aberration that occurs in the optical systemof the optical pickup device 10 is corrected.

However, if the spherical aberration is to be corrected by a sphericalaberration correcting mechanism, the distance between the first element4 and the second element 5 of the two-element object lens 9 may be fixedwhile the spherical aberration is corrected in accordance with a valueof the spherical aberration error signal SAES supplied to the sphericalaberration correcting mechanism.

By using the signals supplied from the detecting devices 7 and 8, theinformation reproducing circuit 36 reproduces information stored in theoptical disk 6, so as to generate reproduction signals.

With reference to FIG. 2, details of the optical pickup device 10 aredescribed below. For the purpose of easy explanation, the mirror 27shown in FIG. 1 is omitted in the optical pickup device 10 shown in FIG.2.

The optical disk 6 is an optical storing medium. As shown in FIG. 2, theoptical disk 6 includes a cover glass 6 a, a substrate 6 b, and twoinformation storing layers 6 c and 6 d. The information storing layers 6c and 6 d are provided between the cover glass 6 a and the substrate 6b. Thus, the optical disk 6 is a two-layered disk. The optical pickupdevice 10 focuses a light beam on either one of the information storinglayers 6 c and 6 d. In this way, information is reproduced from theinformation storing layer, or information is recorded in the informationstoring layer.

Therefore, in the following description, the information storing layerof the optical disk 6 is either one of the information storing layer 6 cand 6 d. The optical pickup device may focus a light beam on any one ofthe information storing layers, so as to record or reproduceinformation.

In the optical pickup device 10, disposed on an optical axis OZ formedbetween a light beam radiating surface of the semiconductor laser 1 anda light beam reflecting surface of the optical disk 6 are the hologram2, the collimating lens 3, and the two-element object lens 9 includingthe first element 4 and the second element 5. The detecting devices 7and 8 are disposed in focusing positions of the light beams that havebeen diffracted by the hologram 2.

Specifically, in the optical pickup device 10 having the foregoingarrangement, the light beam radiated from the semiconductor laser 1 istransmitted as a zero-order diffracted light beam through the hologram2. Then, the light beam is converted by the collimating lens 3 intoparallel light. After that, the light beam is transmitted through thetwo-element object lens 9 including the first element 4 and the secondelement 5, which are two lenses. Finally, the light beam is focused onthe information storing layer (information storing layer 6 c or 6 d) onthe optical disk 6.

On the other hand, the light beam reflected from the information storinglayer is incident on the hologram 2 via the second element 5 and thefirst element 4 of the two-element object lens 9, and the collimatinglens 3, in this order. Then, the light beam is diffracted by thehologram 2, and focused on the detecting devices 7 and 8.

The detecting device 7 includes a first light receiving section 7 a anda second light receiving section 7 b. The detecting device 8 includes athird light receiving section 8 a and a fourth light receiving section 8b. The detecting devices 7 and 8 convert, into electrical signals, thelight beams focused.

Next, an arrangement of the hologram 2 is described. The hologram 2includes four regions 2 a, 2 b, 2 c, and 2 d. The hologram 2 is dividedinto two regions by a straight line CL1, which is a divisional line. Thetwo regions are a semicircular region including the regions 2 c and 2 d,and a semicircular region including the regions 2 a and 2 b.

One of the two regions formed by division, that is, the semicircularregion that includes the regions 2 c and 2 d, is divided into tworegions by a straight line CL2, which is a divisional line. The tworegions are the regions 2 c and 2 d. On the other hand, the semicircularregion that includes the regions 2 a and 2 b is divided into two regionsby an arc E2, which is an arc-shaped divisional line. The two regionsare the regions 2 a and 2 b. The region 2 a corresponds to a highnumerical aperture region of the two-element object lens 9. The region 2b corresponds to a low numerical aperture region of the two-elementobject lens 9. Therefore, the light beam is split by the regions 2 a and2 b into an outer circumferential region and an inner circumferentialregion.

Thus, the region 2 a is a region surrounded by the straight line CL1orthogonal to the optical axis OZ, and by an arc E1 and an arc E2, bothof which center on the optical axis OZ. The region 2 b is a regionsurrounded by the straight line CL1 and the arc E2.

The region 2 c is a region surrounded by the straight line CL1, thestraight line CL2 orthogonal to the straight line CL1, and by the arcE1. Like the region 2 c, the region 2 d is a region surrounded by thestraight line CL1, the straight line CL2, and the arc E1.

Through the hologram 2, the light beam emitted from a semiconductorlaser 1 side is transmitted, as zero-order diffracted light beams, to anoptical disk 6 side. Then, reflected light beams from the optical disk 6side are diffracted by the hologram 2, so as to lead into the detectingdevices 7 and 8.

Through the hologram 2, the light beams from the optical disk 6 side istransmitted and diffracted, so that the light beams from the opticaldisk 6 side are respectively focused on different points according tothe regions through which the beams are transmitted and diffracted.

Specifically, among the light beams reflected on the information storinglayers of the optical disk 6, a first light beam diffracted by theregion 2 a of the hologram 2 forms a focusing spot on the first lightreceiving section 7 a. A second light beam diffracted by the region 2 bof the hologram 2 forms a focusing spot on the second light receivingsection 7 b. A third light beam diffracted by the region 2 c of thehologram 2 forms a focusing spot on the third light receiving section 8a. A fourth light beam diffracted by the region 2 d of the hologram 2forms a focusing spot on the fourth light receiving section 8 b.

Now, with reference to FIG. 3, details of the detecting devices 7 and 8are described.

As shown in FIG. 3, the detecting device 7 includes the two lightreceiving sections (the first light receiving section 7 a and the secondlight receiving section 7 b). The detecting device 8 includes the twolight receiving sections (the third light receiving section 8 a and thefourth light receiving section 8 b).

Furthermore, the first light receiving section 7 a is divided into two:light detectors 11 a and 11 b. The second light receiving section 7 b isdivided into two: light detectors 12 a and 12 b. The light receivingsections are disposed so that the focusing spots of the first light beamand the second light beam are respectively formed on divisional lines ofthe light receiving sections. The light receiving sections convert thelight beams into electrical signals.

The third light receiving section 8 a includes a light detector 13, andconverts the third light beam into an electrical signal. The fourthlight receiving section 8 b includes a light detector 14, and convertsthe fourth light beam into an electrical signal.

The electrical signals obtained at the light detectors are used, by thedrive control section 30 (see FIG. 1), for the focal point displacementof the two-element object lens 9 or for reproducing information from theoptical disk 6. For example, the electrical signals are supplied to theinformation reproducing circuit 36 (see FIG. 1), and converted into RFsignals (reproduction signals). Here, the RF signals stored in theoptical disk 6 are given by a sum of the electrical signals suppliedfrom the light detectors.

Incidentally, in the optical recording and reproducing apparatus havingthe foregoing arrangement, tracking drive control is performed so as tofocus, on a track formed on the optical disk 6, the light beam emittedfrom the two-element object lens 9. That is, the two-element object lens9 is moved in the radial direction of the optical disk 6 by driving thetracking actuator 26 (see FIG. 1), so that the light beam is focused ona track.

Here, a tracking error signal TES, which represents an amount of a gap(tracking error) by which the focal point of the light beam is displacedin the radial direction from the track, is given byTES=14S−13S  (2)where 13S is an electrical signal supplied from the light detector 13,and 14S is an electrical signal supplied from the light detector 14.

A method of measuring the tracking error by calculating the trackingerror signal TES from Formula (2) is called a push-pull method. In thepush-pull method, used is a phenomenon that a pattern of a reflected anddiffracted light beam is unbalanced in the radial direction due to apositional relationship between the track and the focal point (focusingspot) of the light beam. In order to measure an amount of the unbalance(amount of a gap), it is preferable that the straight line CL2, which isa divisional line separating the region 2 c and the region 2 d of thehologram 2, is orthogonal to the radial direction.

The following describes how to detect and correct a focus error (focalpoint displacement in the direction of the optical axis) of thetwo-element object lens 9 by using the electrical signals supplied fromthe light detectors.

When the focal point is not in the information storing layer, the lightbeam on the first light receiving section 7 a is shifted toward one ofthe light detectors of the first light receiving section 7 a, and thelight beam on, the second light receiving section 7 b is shifted towardone of the light detectors of the second light receiving section 7 b.Therefore, a first error signal F1 (first focus error signal obtained bydetecting the focal point displacement which occurs in the direction ofthe optical axis, of the light beam in the outer circumferential region)is given byF1=11aS−11bS  (3)where 11 aS is an electrical signal supplied from the light detector 11a that converts, into the electrical signal, the diffracted light beamsupplied from the region 2 a of the hologram 2, and 11 bS is anelectrical signal supplied from the light detector 11 b that converts,into the electrical signal, the diffracted light beam supplied from theregion 2 a of the hologram 2. A second error signal F2 (second focuserror signal obtained by detecting the focal point displacement, whichoccurs in the direction of the optical axis, of the light beam in theinner circumferential region) is given byF2=12aS−12bS  (4)where 12 aS is an electrical signal supplied from the light detector 12a that converts, into the electrical signal, the diffracted light beamsupplied from the region 2 b of the hologram 2, and 12 bS is anelectrical signal supplied from the light detector 12 b that converts,into the electrical signal, the diffracted light beam supplied from theregion 2 b of the hologram 2.

If the focal point is not in the information storing layer, outputvalues of the error signals F1 and F2 correspond to amounts of the focalpoint displacement that occurs in the direction of the optical axis.Here, the focal point displacement (focus error) is an amount ofdeviation, in the direction of the optical axis, between (1) the focalpoint on which the light beam passing through the two-element objectlens 9 from the semiconductor laser 1 side is focused, and (2) aposition of the information storing layer of the optical disk 6.

Therefore, in order to keep the focal point always in the informationstoring layer, the two-element object lens 9 is moved in the directionof the optical axis OZ in such a manner that the outputs of the firsterror signal F1 or the second error signal F2 are always zero.

Note that, although the above-described method for detecting the focalpoint displacement is a method generally called a “knife edge method”,the method for detecting the focal point displacement is not limited tothe knife edge method. For example, a beam size method may be employed,in which the focal point displacement is detected from a change of abeam size caused by the focal point displacement. In the followingdescription, the knife edge method is employed.

Usually, the focus error signal FES is detected by using an entireeffective diameter of the light beam. Therefore, in the presentembodiment, the focus error signal FES is given byFES=F1+F2  (5).

The following describes how to detect the spherical aberration thatoccurs in the two-element object lens 9, which is the focusing opticalsystem.

When the two-element object lens 9 is used, spherical aberration iscaused by, for example, a variation of the thickness of the cover glass6 a of the optical disk 6. The spherical aberration causes offset in thefocus error signal FES. Therefore, there is a possibility thatinformation cannot be recorded or reproduced, because there is apossibility that the light beam does not match a best image point on theinformation storing layer even if the output of the focus error signalFES detected is zero. Here, the best image point is a position of animage point at which the beam diameter of the light beam is minimized.

If spherical aberration is caused, focal points vary within the lightbeam. Therefore, the spherical aberration can be detected by detecting adifference between a focal point in the inner circumferential region ofthe light beam or a focal point in the outer circumferential region ofthe light beam. That is, the spherical aberration error signal SAES isgiven by one of the following formulas (6) to (8):SAES=F1  (6)SAES=F2  (7)SAES=F1−F2  (8).Thus, the spherical aberration error signal SAES can be detected byusing the first error signal F1 or the second error signal F2.

As described above, the spherical aberration is detected from the focuserror signal obtained from at least one of the inner circumferentialregion of the light beam and the outer circumferential region of thelight beam, which are obtained by separation performed at the regions 2a and 2 b of the hologram 2.

In this way, it is possible to detect the spherical aberrationsensitively.

Next, a procedure of drive control for the two-element object lens 9 isdescribed, with reference to the flowchart in FIG. 4.

First, as shown in FIG. 4, the focus error signal FES is generated inthe control signal generating circuit 35 of the drive control section 30in accordance with the electrical signal supplied from the opticalpickup device 10. That is, the focus error signal FES is detected by theoptical pickup device 10 (S1). Then, a focus servo loop is turned ON,and focus control (focus control process) is performed by using thefocus drive circuit 33 and the focus actuator 23, so that a value of thefocus error signal FES becomes equal to or close to zero (S2).

Next, the spherical aberration error signal SAES is detected in thecontrol signal generating circuit 35 in accordance with the electricalsignal supplied from the optical pickup device 10 (S3). Then, a loop ofa spherical aberration correcting servo is turned ON, and sphericalaberration correcting control (spherical aberration correction process)is performed in accordance with the spherical aberration error signalSAES by using the second element drive circuit 32 and the actuator 25,so that the spherical aberration is corrected, and a value of thespherical aberration error signal SAES becomes equal to or close to zero(S4).

Subsequently, the tracking error signal TES is detected in the controlsignal generating circuit 35, in accordance with the electrical signalsupplied from the optical pickup device 10 (S5). Then, by using thefocus drive circuit 33 and the tracking actuator 26, the a loop of atracking error servo is turned ON, and tracking control is performed, sothat a value of the tracking error signal TES becomes equal to or closeto zero (S6).

After S1 to S6 are performed, offset adjustment (focus offset adjustmentprocess) for the focus error signal FES is started (S7).

Here, the control signal generating circuit 35 monitors an amplitude ofthe RF signal, and outputs a monitoring result to the focus drivecircuit 33.

After that, by driving the focus actuator 23 in accordance with themonitoring result, the two-element object lens 9 is moved toward theoptical disk 6 or away from the optical disk 6, so that the focal pointis adjusted to maximize the amplitude of the RF signal (S8). In thisway, a just focal point, at which the amplitude of the RF signal ismaximized, is determined, and the offset adjustment for the focus errorsignal FES is terminated (S9).

Thus, it is preferable that the two-element object lens 9 is a lensgroup including one or more lens (here, the first element 4 and thesecond element 5), and that the spherical aberration is corrected bymoving at least one lens (the second element 5) of the two-elementobject lens 9.

With this arrangement, it is possible to correct the sphericalaberration with a simple arrangement and with high accuracy.

Here, described below with reference to the flowchart of FIG. 5 is acomparative example in which the spherical aberration is corrected andthe loop of the spherical aberration correcting servo is turned ON afterthe offset of the focus error signal FES is adjusted.

In this comparative example, first, the focus error signal FES isdetected by the optical pickup device 10 (S11 (equivalent to S1)). Then,the focus servo loop is turned ON, and focus control is performed, sothat a value of the focus error signal FES becomes equal to or close tozero (S12 (equivalent to S2)).

Next, the tracking error signal TES is detected by the optical pickupdevice 10 (S13 (equivalent to S5)). Then, the loop of the tracking errorservo is turned ON, and tracking control is performed, so that a valueof the tracking error signal TES becomes equal to or close to zero (S14(equivalent to S6)).

After that, offset adjustment for the focus error signal FES is started(S15 (equivalent to S7)). By driving the focus actuator 23, the focalpoint is adjusted so that the amplitude of the RF signal is maximized(S16 (equivalent to S8)). In this way, a just focal point, at which theamplitude of the RF signal is maximized, is determined, and the offsetadjustment for the focus error signal FES is terminated (S17 (equivalentto S9)).

Thereafter, the spherical aberration error signal SAES is detected (S18(equivalent to S3)). Then, the loop of the spherical aberrationcorrecting servo is turned ON, and spherical aberration correctingcontrol is performed, so that a value of the spherical aberration errorsignal SAES becomes equal to or close to zero (S19 (equivalent to S4)).

The offset adjustment for the focus error signal FES in such a case isdescribed with reference to FIGS. 6( a) and 6(b).

FIG. 6( a) is a graph illustrating the focus error signal FES before thespherical aberration is corrected. In FIG. 6( a), the point O indicatesa case in which a defocus amount of the two-element object lens 9 iszero. The defocus amount is zero when the just focal point of the lightbeam matches the information storing layer of the optical disk 6.

Here, as shown in FIG. 6( a), the focus error signal FES before thespherical aberration is corrected is such that the point A, at which thefocus error signal FES is zero, does not match the point O, at which thedefocus amount is zero. Therefore, there is offset of an amount from thepoint A to the point O. Described below is a case in which a servosystem introduction movement is performed under this condition.

First, when the focus error signal FES is detected and the loop of thefocus servo is turned ON, the focus actuator 23 drives the two-elementobject lens 9, so that the focus error signal FES becomes zero. That is,the point A is the target of the focus control.

Next, not the correction of the spherical aberration, but the offsetadjustment for the focus error signal FES is performed. Actually, whilethe RF signal is monitored, the focus actuator 23 is driven so that theamplitude of the RF signal is maximized. When the offset adjustment forthe focus error signal FES is thus performed, the focus actuator 23performs focus control targeted at an output of the point B. Whenspherical aberration is corrected under an offset-free state after theoffset adjustment for the focus error signal FES is performed, the focuserror signal FES looks as in FIG. 6( b).

Usually, when spherical aberration is corrected, a slope of a linearportion of the focus error signal FES becomes steep. That is, thespherical aberration of the two-element object lens 9 becomes so smallas to be negligible. Therefore, the focus error signal FES has highsensitivity.

However, a just focus is no longer attained at the point B, because theslope of the linear portion of the focus error signal FES is steep.Therefore, when focus control targeted at the point B is performed, theoffset of an amount from the point A to the point O is left.

As a result, if, as shown in FIG. 5, the spherical aberration iscorrected and the loop of the spherical aberration correcting servo isturned ON after the offset of the focus error signal FES is adjusted,offset cannot be eliminated from the focus error signal FES.

On the other hand, if the offset of the focus error signal FES isadjusted after the spherical aberration is corrected, the focus actuator23 is driven so that the offset becomes zero after the slope of thelinear portion of the focus error signal FES becomes steep.

Thus, by adjusting the offset of the focus error signal FES after theamount of the spherical aberration is reduced as much as possible bycorrecting the spherical aberration and turning ON the loop of thespherical aberration correcting servo, it is possible to eliminateoffset from the focus error signal FES.

As described above, in the present embodiment, the spherical aberrationis corrected by, for example, moving the second element 5, so as tochange a distance between the first element 4 and the second element 5,which constitute the two-element object lens 9. That is, in order tocorrect the spherical aberration, the light beam is caused to diverge orconverge, so that a divergent or convergent light beam is incident intothe optical disk 6.

However, usually, when a divergent or convergent light beam is incident,a magnification of the optical system is changed, so that a sensitivityof the focus error signal FES is changed. Therefore, if the sphericalaberration is corrected after the offset adjustment for the focus errorsignal FES is performed, the sensitivity of the focus error signal FESis changed, thereby leaving offset.

On the other hand, what is carried out in the offset adjustment for thefocus error signal FES is merely adjusting the focal point displacementby driving the focus actuator 23 in the direction of the optical axis.This driving causes no spherical aberration in the optical system.Therefore, the offset adjustment for the focus error signal FES causesno additional spherical aberration in the optical system.

Therefore, it is preferable that, as in an adjustment procedure in thefocal point adjusting method shown in FIG. 4, information is recorded inor reproduced from the optical recording medium after the optical pickupdevice is adjusted, so that no focus error is left therein, byperforming the offset adjustment for the focus error signal FES afterthe spherical aberration is corrected.

As described above, the focal point adjusting method of the presentinvention for adjusting the focal point of the focused light beamincludes:

a focus controlling process in which an output of the focus error signalFES is controlled so that the output becomes close to zero, the focuserror signal FES being obtained by detecting focal point displacementthat occurs in the direction of the optical axis of the light beamfocused by passing through the two-element object lens 9;

a spherical aberration correcting process in which spherical aberrationthat occurs with respect to the light beam is corrected; and

a focus offset adjusting process in which offset in the focus errorsignal FES is adjusted.

Therefore, the optical pickup device 10 employing the focal pointadjusting method includes:

the semiconductor laser 1;

the two-element object lens 9 that focuses a light beam radiated fromthe semiconductor laser 1 and reflected on the optical disk 6;

the control signal generating circuit 35, which is focus error detectingmeans for detecting the focus error signal FES indicative of the focalpoint displacement that occurs in the direction of the optical axis ofthe light beam, and which detect spherical aberration of the two-elementobject lens 9;

the focus drive circuit 33 and the focus actuator 23, which control anoutput of the focus error signal FES so that the output becomes close tozero, and which are focus offset control means for adjusting offset ofthe focus error signal FES; and

the actuator 25 and the second element drive circuit 32 for correctingspherical aberration in accordance with the focus error signal FES,

the focus drive circuit 33 and the focus actuator 23, which are thefocusing offset adjusting means, controlling the output of the focuserror signal FES so that the output becomes close to zero, and adjustingthe offset of the focus error signal FES after the spherical aberrationis corrected by the actuator 25 and the second element drive circuit 32.

According to this arrangement, the offset of the focus error signal FESis adjusted after (1) the output of the focus error signal is controlledto be zero in the focus controlling process, and (2) the sphericalaberration in the two-element object lens 9 is reduced to be negligibleby steepening the slope of the linear portion of the focus error signalFES.

Thus, if the offset of the focus error signal FES is adjusted after thespherical aberration is corrected with respect to the focus error signalFES whose output is zero, the offset is adjusted to be zero after theslope of the linear portion of the focus error signal is steepened.

Therefore, by adjusting the offset of the focus error signal FES afterthe amount of the spherical aberration that occurs in the two-elementobject lens 9 is reduced as much as possible by (1) correcting thespherical aberration, and, for example, (2) turning ON the loop of thespherical aberration correcting servo, it is possible to eliminateoffset from the focus error signal FES.

In this way, it is possible to provide a focal point adjustment methodwith which the spherical aberration is corrected stably, and the focalpoint displacement that occurs in the direction of the optical axis iscontrolled stably, so that, for example, the focal point of the lightbeam radiated is not displaced, and to provide an optical pickup device10 employing the focal point adjusting method.

Incidentally, as shown in Formulas (6) to (8), in the sphericalaberration error signal SAES, the first error signal F1 obtained in thefirst light receiving section 7 a of the detecting device 7, or thesecond error signal F2 obtained in the second light receiving section 7b of the detecting device 7, is used. In other words, the focus errorsignal in the inner circumferential portion of the light beam or thefocus error signal in the outer circumferential portion of the lightbeam is used in the spherical aberration error signal SAES.

Thus, by performing the offset adjustment for the focus error signalFES, offset is caused in the spherical aberration error signal SAES.Therefore, from the spherical aberration error signal SAES, it isnecessary to remove crosstalk from the focus error signal FES.

In such a case, it is preferable to correct the spherical aberrationerror signal SAES by using the focus error signal FES, so as to removethe crosstalk of the focus error signal FES.

That is, it is preferable that the spherical aberration error signalSAES is given bySAES=F1−(F1+F2)×K1  (9),where K1 is a coefficient, orSAES=F2−(F1+F2)×K2  (10),where K2 is a coefficient.

With this setting, it is possible to remove, from the sphericalaberration error signal SAES, the crosstalk from the focus error signalFES.

Moreover, the spherical aberration error signal SAES may be given bySAES=F1−F2×K3  (11),where K3 is a coefficient.

With this setting also, it is possible to remove, from the sphericalaberration error signal SAES, the crosstalk from the focus error signalFES. Therefore, it is possible to accurately detect the sphericalaberration from the spherical aberration error signal SAES.

Here, K1, K2, and K3 may be decided so that the crosstalk from the focuserror signal FES becomes small.

By monitoring the RF signal, the offset of the focus error signal FES isso adjusted that the amplitude of the RF signal is maximized. Note that,for example, in case of an unrecorded optical disk 6, offset can beadjusted by using an RF signal modulated at a pit in an address section.

Moreover, there are cases in which the RF signal cannot be used for theoffset adjustment because no modulated component appears in the RFsignal of the unrecorded optical disk 6, such as a case in which theoptical disk 6 has a wobble structure, that is, if the optical disk 6has undulating track grooves, instead of a pit in the address section,so as to store address information. In such a case, the tracking errorsignal TES may be used instead of the RF signal, so that the offsetadjustment for the focus error signal FES is performed by using theamplitude of the tracking error signal TES.

FIG. 7 illustrates a focal point adjusting method of this case.

First, the focus error signal FES is detected (S21). Then, the focusservo loop is turned ON, and focus control is performed, so that a valueof the focus error signal FES becomes equal to or close to zero (S22).

Next, the spherical aberration error signal SAES from the optical pickupdevice 10 is detected (S23). Then, the loop of the spherical aberrationcorrecting servo is turned ON, and spherical aberration correctingcontrol is performed, so that a value of the spherical aberration errorsignal SAES becomes equal to or close to zero (S24).

After that, the offset adjustment for the focus error signal FES isstarted (S25). Then, by driving the focus actuator 23, the focal pointis adjusted until the amplitude of the tracking error signal TES ismaximized (S26).

In this way, a just focal point, at which the amplitude of the trackingerror signal TES is maximized, is determined, and the offset adjustmentfor the focus error signal FES is terminated (S27).

Thereafter, the tracking error signal TES from the optical pickup device10 is detected (S28). Then, the loop of the tracking error servo isturned ON, and tracking control is performed, so that a value of thetracking error signal TES becomes equal to or close to zero (S29).

By thus adjusting the offset of the focus error signal FES after thespherical aberration is corrected, it is possible to eliminate offsetfrom the focus error signal FES even if the offset of the focus errorsignal FES is adjusted before turning ON the loop of the tracking errorservo (i.e. before the tracking control).

Moreover, in case the optical disk 6 has the wobble structure, theoffset of the focus error signal FES may be adjusted by using anamplitude of a wobble signal. In such a case, after the tracking servoloop is turned ON, the wobble signal is detected from the tracking errorsignal TES, and the offset of the focus error signal FES is adjusted sothat the amplitude of the wobble signal is maximized.

In case the optical disk 6 has a plurality of information storinglayers, it is ensured, not only when the optical disk 6 is loaded (diskload time), but also when the focal point jumps from one of theinformation storing layers into another of the information storinglayers (inter-layer jump time), that offset is not left in the focuserror signal FES if the offset adjustment is performed according to theforegoing procedure (i.e. if the offset adjustment is performed aftercorrecting the spherical aberration and turning ON the loop of thespherical aberration correcting servo).

Moreover, in case an inter-layer jump is performed among a plurality ofinformation storing layers, it may be so arranged that (1) sphericalaberration that occurs due to thickness of the information storinglayers is corrected in advance, and an amount of correction of thespherical aberration is finely adjusted after the jump, or (2) thespherical aberration is not corrected before the jump but sphericalaberration that occurs due to the thickness of the information storinglayers and due to unevenness of the thickness of the optical disk 6 iscorrected after the jump.

Incidentally, as described above, in the optical pickup device 10, thefocus error signal FES is used as the spherical aberration error signal.That is, as a reference signal for the spherical aberration correction,the focus position displacement between the light beam in the innercircumferential portion and the light beam in the outer circumferentialportion is used.

This aims at correcting, in real time when information is recorded orreproduced, the spherical aberration that occurs due to the unevennessof the thickness of the optical disk 6 (which is an optical storingmedium) if the unevenness of the thickness is significant.

However, depending on manufacturing art for the optical storing medium,the unevenness of the thickness of the optical disk 6 can be kept lesssignificant.

If the unevenness of the thickness of the optical disk 6 is kept lesssignificant, it is sufficient that the spherical aberration that occursdue to the unevenness of the thickness of the optical disk 6 iscorrected only in the disk load time and the inter-layer jump time. Thatis, it is not necessary to perform the spherical aberration correctionin real time.

If the spherical aberration is corrected only in the disk load time andthe inter-layer jump, time, the amplitude of the RF signal or theamplitude of the tracking error signal TES can be used as a referencesignal indicative of the amount of the spherical aberration.

By thus using the amplitude of the RF signal or the amplitude of thetracking error signal TES in performing the spherical aberrationcorrection, it is possible to use an optical system having anarrangement simpler than that of a case in which the sphericalaberration is corrected in real time.

With reference to the flowchart in FIG. 8, the following describes anexample of a procedure of drive control for the two-element object lens9 for a case in which the RF signal is used for correcting the sphericalaberration.

First, the focus error signal FES is detected (S31). Then, the focusservo loop is turned ON, and focus control is performed, so that thevalue of the focus error signal FES becomes equal to or close to zero(S32).

Next, the tracking error signal TES is detected (S33). Then, thetracking servo loop is turned ON, and tracking control is performed, sothat the value of the tracking error signal TES becomes equal to orclose to zero (S34).

Then, while the amplitude of the RF signal is monitored by the controlsignal generating circuit 35, the spherical aberration correction isstarted (S35). That is, by using the actuator 25, the distance betweenthe first element 4 and the second element 5, which constitute thetwo-element object lens 9, is changed, so as to correct the sphericalaberration in such a manner that the amplitude of the RF signal ismaximized (S36).

Thereafter, while the amplitude of the RF signal is monitoredcontinually, the offset adjustment for the focus error signal FES isstarted (S37). That is, the control signal generating circuit 35monitors the amplitude of the RF signal, and supplies a monitoringresult to the focus drive circuit 33.

Then, by driving the focus actuator 23 in accordance with the monitoringresult, the two-element object lens 9 is moved toward the optical disk 6or away from the optical disk 6, so that the focal point is adjusted tomaximize the amplitude of the RF signal. In this way, the just focalpoint, at which the amplitude of the RF signal is maximized, isdetermined, and the offset adjustment for the focus error signal FES isterminated (S38). Here, the adjustment is terminated as a whole.

With reference to the flowchart in FIG. 9, the following describes anexample of a procedure of driving control for the two-element objectlens 9 for a case in which the tracking error signal TES is used forcorrecting the spherical aberration.

First, the focus error signal FES is detected (S41). Then, the focusservo loop is turned ON, and focus control is performed, so that thevalue of the focus error signal FES becomes equal to or close to zero(S42).

Then, while the amplitude of tracking error signal TES is monitored bythe control signal generating circuit 35, correction of the sphericalaberration is started (S43). That is, by using the actuator 25, thedistance between the first element 4 and the second element 5, whichconstitute the two-element object lens 9, is changed, so as to correctthe spherical aberration in such a manner that the amplitude of thetracking error signal TES is maximized (S44).

Thereafter, while the amplitude of the tracking error signal TES ismonitored continually, the offset adjustment for the focus error signalFES is started (S45). That is, the control signal generating circuit 35monitors the amplitude of the tracking error signal TES, and supplies amonitoring result to the focus drive circuit 33.

Then, by driving the focus actuator 23 in accordance with the monitoringresult, the two-element object lens 9 is moved toward the optical disk 6or away from the optical disk 6, so that the focal point is adjusted tomaximize the amplitude of the tracking error signal TES (S46).

Then, the tracking error signal TES is detected (S47). After that, theloop of the tracking servo is turned ON, and tracking control isperformed, so that the value of the tracking error signal TES becomesequal to or close to zero (S48). Here, the adjustment is terminated.

Note that the spherical aberration correction may be performed byemploying a method called SAM (Sequenced Amplitude Margin). In SAM, usedis a path metric difference in a Viterbi decoding, which is one of themethods of evaluating quality of a signal as a reference signalindicative of the amount of the spherical aberration.

In the foregoing focal point adjusting method in which sphericalaberration correction is performed only at the disk load time and at theinter-layer jump time, correction of the spherical aberration and theoffset adjustment for the focus error signal FES are performed onlyonce, respectively. However, the focal point adjusting method is notlimited to this arrangement. For example, if the offset amount of thefocus error signal FES or a remaining amount of spherical aberration isinitially (i.e. before focal point adjustment is performed) large, thereare cases in which quality of the RF signal is not sufficiently improvedeven if the correction of spherical aberration and the offset adjustmentfor the focus error signal FES are performed once, respectively.

In such cases, the loop of the correction of spherical aberration andthe loop of the offset adjustment for the focus error signal FES may berespectively executed plural times with an aim of improving accuracy ofthe adjustment, so that the quality of the RF signal is improved. Notethat it is preferable to perform the offset adjustment for the focuserror signal FES at the end of the loop, because the sensitivity of thefocus error signal is changed by the spherical aberration correctionalso in this case.

In the present embodiment, the two-element object lens 9 including thetwo lenses (the first element 4 and the second element 5) is used as theobject lens. However, the object lens may be a single lens, so that theapparatus can be assembled in simple steps.

In the present embodiment, spherical aberration is corrected by changingthe distance between the first element 4 and the second element 5, whichconstitute the two-element object lens 9. However, the present inventionis not limited to this arrangement. For example, the collimating lens 3may be moved so as to change a distance between the semiconductor laser1 and the collimating lens 3. In this case, the light beam emitted fromthe semiconductor laser 1 and transmitted through the collimating lens 3becomes non-parallel, thereby generating spherical aberration. By usingthe spherical aberration, the spherical aberration in the optical systemof the optical pickup device 10, that is, the spherical aberration inthe two-element object lens 9, can be corrected.

Moreover, a spherical aberration correcting mechanism may be insertedbetween the two-element object lens 9 and the collimating lens 3. Thespherical aberration correcting mechanism constitutes an optical systemthat causes spherical aberration when a light beam is transmittedthrough the spherical aberration correcting mechanism.

For example, as the spherical aberration correcting mechanism, an afocaloptical system made by combining a convex lens having positive power anda concave lens having negative power may be used. By adjusting adistance between the convex lens and the concave lens, it is possible tocause spherical aberration. Moreover, as another arrangement of thespherical aberration correcting mechanism, an afocal optical system madeby combining two convex lenses respectively having positive power may beused. In this case also, it is possible to cause spherical aberration byadjusting a distance between the two convex lenses. Moreover, as thespherical aberration correcting mechanism for causing sphericalaberration, a liquid crystal element having spherical aberration may beused.

By thus providing the spherical aberration correcting mechanism, it ispossible to correct the spherical aberration in the two-element objectlens 9 by using the spherical aberration caused by the sphericalaberration correcting mechanism.

Note that the specific embodiments and examples described in the “BESTMODE FOR CARRYING OUT THE INVENTION” section are only for clarifyingtechnical contents of the present invention. Therefore, the scope of thepresent invention should not be interpreted as being limited to thesespecific examples. The present invention may be varied in many wayswithin the scope of the spirits of the present invention and thefollowing claims.

INDUSTRIAL APPLICABILITY

According to the arrangement or method of the present invention,spherical aberration is corrected stably, and focal point displacementcontrol in the direction of the optical axis can be performed stably, sothat the focal point of the light beam radiated is not displaced.Therefore, the present invention is suitable for use in a focal pointadjusting method in which the focal point displacement that occurs inthe focusing optical system is detected and the focal point is adjusted,and for use in an optical pickup device employing the focal pointadjusting method.

1. A focal point adjusting method for adjusting a focal point of afocused light beam, comprising: a focus controlling process in which anoutput of a focus error signal is controlled so that the output becomesclose to zero, the focus error signal being obtained by detecting focalpoint displacement that occurs in a direction of an optical axis of thelight beam focused by passing through a focusing optical system; aspherical aberration correcting process in which spherical aberrationthat occurs with respect to the focused light beam is corrected; and afocus offset adjusting process in which offset in the focus error signalis determined and adjusted, said focus offset adjusting process beingperformed after said spherical aberration correcting process correctsfor spherical aberration.
 2. The focal point adjusting method as setforth in claim 1, wherein: the spherical aberration is detected from afocus error signal obtained from at least one of (a) an innercircumferential region of a reflected light beam and (b) an outercircumferential region of the reflected light beam, which are split fromeach other by a light beam splitting means.
 3. The focal point adjustingmethod as set forth in claim 2, wherein: a spherical aberration errorsignal SAES, which is indicative of the spherical aberration, satisfiesany one of the following:SAES=F1−(F1+F2)×K1;SABS=F2−(F1+F2)×K2; andSAES=F1−F2×K3, where F1 is a first focus error signal obtained bydetecting focal point displacement, which occurs in the direction of theoptical axis, of the outer circumferential region of the reflected lightbeam, F2 is a second focus error signal obtained by detecting the focalpoint displacement, which occurs in the direction of the optical axis,of the inner circumferential region of the reflected light beam, and K1K2, and K3 are coefficients.
 4. The focal point adjusting method as setforth in claim 1, wherein: in the spherical aberration correctingprocess, the spherical aberration of the focusing optical system iscorrected by moving at least one lens of a lens group including one ormore lens of the focusing optical system.
 5. The focal point adjustingmethod as set forth in claim 1, further comprising: repeating each ofsaid spherical aberration correcting process and said focus offsetadjusting process; and terminating said repeating after the focus offsetadjustment process is performed at an end of the repetition.
 6. Thefocal point adjusting method as set forth in claim 1, wherein said focusoffset adjusting process is performed after both the focus error signalbecomes close to zero and after said spherical aberration correctingprocess corrects for spherical aberration.
 7. An optical pickup device,comprising: a light source; a focusing optical system that focuses alight beam radiated from the light source and which transmits anotherlight beam that is reflected from a storing medium there through;focusing error detecting means for detecting a focus error signalindicative of focal point displacement that occurs in a direction of anoptical axis of the focused light beam; focus control means forcontrolling an output of the focus error signal so that the outputbecomes close to zero; focus offset adjusting means for adjusting offsetof the focus error signal; spherical aberration detecting means fordetecting spherical aberration of the focusing optical system; andspherical aberration correcting means for correcting the sphericalaberration, the focus offset adjusting means for determining andadjusting the offset of the focus error signal after (i) the focuscontrol means controls the output of the focus error signal, and (ii)the spherical aberration correcting means corrects the sphericalaberration.
 8. The optical pickup device as set forth in claim 7,further comprising: light beam splitting means for splitting, into (a)an inner circumferential region and (b) an outer circumferential region,the reflected light beam transmitted through the focusing opticalsystem, the spherical aberration detecting means detecting the sphericalaberration from a focus error signal obtained from at least one of (a)the inner circumferential region of the reflected light beam and (b) theouter circumferential region of the reflected light beam.
 9. The opticalpickup device as set forth in claim 8, wherein: the spherical aberrationdetecting means generates a spherical aberration error signal indicativeof the spherical aberration of the focusing optical system; and SAES,which is the spherical aberration error signal, satisfies any one of thefollowing:SAES=F1−(F1+F2)×K1;SAES=F2−(F1+F2)×K2; andSAES=F1−F2×K3, where F1 is a first focus error signal obtained bydetecting focal point displacement, which occurs in the direction of theoptical axis, of the outer circumferential region of the reflected lightbeam, F2 is a second focus error signal obtained by detecting focalpoint displacement, which occurs in the direction of the optical axis,of the inner circumferential region of the reflected light beam, and K1,K2, and K3 are coefficients.
 10. The optical pickup device as set forthin claim 7, wherein: the spherical aberration correcting means correctsthe spherical aberration by performing such an adjustment as to maximizean amplitude of a reproduction signal obtained by reading informationstored in the storing medium.
 11. The optical pickup device as set forthin claim 7, wherein: the spherical aberration correcting means correctsthe spherical aberration by performing such an adjustment as to maximizean amplitude of a tracking error signal indicative of the focal pointdisplacement that occurs in a radial direction of the storing medium.12. The optical pickup device as set forth in claim 7, wherein: thefocusing optical system is lens group including one or more lens; andthe spherical aberration correcting means moves at least one lens of thelens group.
 13. The optical pickup device as set forth in claim 7,wherein: the focus offset adjusting means adjusts the offset of thefocus error signal by performing such an adjustment as to maximize anamplitude of a reproduction signal obtained by reading informationstored in the storing medium.
 14. The optical pickup device as set forthin claim 13, further comprising: tracking control means for (a)detecting a tracking error signal indicative of focal point displacementtat occurs in a radial direction of the storing medium, and (b)correcting, in accordance with the tracking error signal, the focalpoint displacement that occurs in the radial direction of the storingmedium, the focus offset adjusting means adjusting the offset of thefocus error signal whose focal point displacement tat occurs in theradial direction of the storing medium has been adjusted by the trackingcontrol means.
 15. The optical pickup device as set forth in claim 7,further comprising: tracking control means for (a) detecting a trackingerror signal indicative of focal point displacement that occurs in aradial direction of the storing medium, and (b) correcting, inaccordance with the tracking error signal, the foe-al point displacementthat occurs in the radial direction of the storing medium, the focusoffset adjusting means adjusting the offset of the focus error signal byperforming such an adjustment as to maximize an amplitude of thetracking error signal.
 16. The optical pickup device as set forth inclaim 7, wherein: if (a) the storing medium has a plurality ofinformation storing layers, and (b) information is recorded in andreproduced from the storing medium, the focus offset adjusting meansadjusts the offset when the focal point of the light beam jumps from oneof the information storing layers into another of the informationstoring layers.
 17. The optical pickup device as set forth in claim 7,wherein: if there is a repetition of (a) an offset adjustment for thefocus error signal performed by the focus offset adjusting means and (b)a correction of the spherical aberration, the focus offset adjustingmeans performs, at an end of the repetition, (a) the offset adjustmentfor the focus error signal.